The invention relates to a method for measuring the resistance and the inductance of a line. Such methods are used e.g. for short circuit detection in electricity supply lines.
In order to measure the impedance and, in particular, the resistance and the inductance of a line, the current flowing in it and the voltage applied across it are generally measured in a time-resolved fashion and converted into the required values by computation. For the measurement of currents, especially in the case of electrical power lines, use has also recently been made of a Rogowski coil, i.e. a coil that extends around the line carrying the current and measures the derivative of the current with respect to time. In order to find the current, this derivative needs to be integrated with respect to time. This necessitates additional numerical calculation and can lead to inaccuracies (clipping effects, phase offset).
It is therefore an object of the invention to provide a method of the type mentioned in the introduction, which gives a maximally accurate and simple way of measuring the inductance and the resistance.
Instead of integrating the values from the Rogowski coil, the measured values are hence substituted directly into an equation system that delivers the desired results.
Preferably, a sizeable number of measurements are used, so that the equation system is over-determined. The resistance and the inductance can then be found by adjustment computation. The method of least squares is preferably employed, which can be done by means of simple matrix inversion or recursively.